This is a competing renewal of a grant that has been funded by the NIH. In the next phase of the project, we plan to continue to achieve the general aims at quantitation of diffusion effects in MR imaging of brain. In the past four years since the beginning of the current funding period, great progress has been made in the understanding of molecular diffusion in tissues, and in the development of new clinical applications of diffusion-weighted MR imaging. New questions, however, have emerged and new techniques have opened up new opportunities for better quantitation and further development. In the next phase we plan to develop new spectroscopy and imaging methods, and concentrate on the following areas: (1) Contributions of blood microcirculation and cytoplasmic streaming to apparent diffusion coefficient (ADC). We will develop methods based on the Velocity Exchange Spectroscopy (VEXSY) (Callaghan et al 1995) to detect water movement of different degrees of correlation and coherence. (2) Changes in ADC with alterations of properties of cellular compartments associated with brain injury. We will use signals from multiple-quantum transition and VEXSY method to detect water diffusion in different environments under controlled cellular morphological changes, and we will measure compartmental tortuosity. (3) Effects of magnetic susceptibility on ADC. New methods will be developed to selectively detect and quantify internal field gradients induced by magnetic susceptibility distributions. (4) Relationship between the ADC and neuronal electrical activities. We will use electrical stimulation of varying duration and intensity to study a rat kindling model of seizures with well characterized anatomic and functional network. We will correlate ADC changes with electrophysiological and immunocytochemistry measurements. (5) diffusion anisotropy changes associated with pathologic conditions or cerebral activity. We will use rat kindling and forepaw stimulation models in electrical stimulation studies, and human volunteers in activation studies. In all these areas, we will continue to use computer simulations to help in the experiment design, validation and quantitation of the results. The project will focus on studies with phantoms, animals, and human volunteers to look at biophysical basis of diffusion and its alteration during brain injury, but the outcomes from these studies should have a direct impact on brain diffusing imaging in a wide range of clinical settings.